1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly to a compound semiconductor device of high operation speed such as a gallium arsenide transistor.
2. Description of the Prior Art
There have been used gallium arsenide semiconductor devices for a high speed signal processing in a high frequency band such as the GHz band.
As shown in FIG. 1(a), there is disposed a gallium arsenide semiconductor chip 3 in a concave portion 2 provided in the central portion of a base 1 made of ceramics such as alumina, and electrodes (not shown) on the gallium arsenide semiconductor chip 3 are connected to terminals formed in a thick film printed circuit patterns 4 on the peripheral portion of the concave portion 2 on the base 1 through bonding wires 5 made of aurum material for example.
However, the surface of the ceramic base 1 for mounting the gallium arsenide chip 3 is so rough that it is difficult to form a circuit with high precision. In order to form circuit patterns on the rough surface of the base, the minimum width of the thick film printed circuit 4 must be approximately 100 .mu.m, making it impossible to form the circuit patterns with a high density. Particularly, since it is difficult to form ground conductors for isolation, impedance matching can not be made, so that the degrees of the freedom of the circuit patterns are few.
Another example of a packaging type of a conventional semiconductor device is shown in FIG. 1(b), in which there are formed thin film circuit patterns 12 on the flat upper surface of a ceramic base 11, and the thin film circuit patterns 12 are connected through wires 14 to a gallium arsenide semiconductor chip 13 which is secured on the surface of the base 11 in its central portion by die-bonding.
Even in this case, however, because of using ceramic material as the base 11, the minimum width of each of the thin film circuit patterns 12 may be decreased only up to approximately 10 .mu.m. While an integral multi layer film circuit patterns can not be formed on the ceramic base 11. Therefore, the arrangement shown in FIG. 1(b) is not suitable for high density mounting.
A further example of a packaging type of a conventional semiconductor device is shown in FIG. 1(c), in which there is die-bonded a gallium arsenide semiconductor chip 23 in the central portion of the upper surface of a silicon substrate 21 having thin film circuit patterns 22 formed on the surface of the silicon substrate 21. The thin film circuit patterns 22 are connected to the gallium arsenide chip 23 through wires 24. In this case, since the flatness of the surface of the silicon substrate 21 is remarkably improved because of using a silicon substrate, the minimum width of the thin film circuit patterns 22 may be approximately 3 .mu.m.
However, in any case as mentioned above, the heat discharging characteristic of the semiconductor device is not inferior. Generally, there is generated a large amount of heat in the semiconductor device during an operation in a high frequency of more than 500 MHz, and since the generated heat can not be sufficiently radiated in the conventional semiconductor device, the frequency characteristic of the semiconductor device is badly effected by the heat, so that the conventional semiconductor device is not suitable to a high frequency operation. In addition, there is a problem that an arrangement for improving the heat radiation characteristic is apt to deteriorate the packing density of the semiconductor device.